The long term goals of this research are to understand the molecular mechanisms underlying the biogenesis, segregation, and cellular localization of centers which regulate the assembly and organization of microtubules in eukaryotic cells. The basal body complex in the unicellular green alga Chlamydomonas is used as a model. The specific aims of this proposal are to further characterize the structure/function relationships of a novel basal body-associated 220 kD Ca2+- binding protein (CaBP). During the previous grant period the protein was identified and purified from basal body complexes isolated from Chlamydomonas reihardtii. In studies on the biochemical properties of the purified protein and the characterization of a cDNA containing the entire coding region for the protein, the 20 kD CaBP was found to be clearly related to the calmodulin family of calcium-binding proteins but distinct from well characterized members of this family in its protein sequence, its cellular localization, and potential functions. There are two major features of the 20 kD CaBP which suggest that elucidation of its structure and function may be of fundamental importance in studies on the molecular mechanisms of cell motility and the function and/or disposition of microtubule organizing centers in eukaryotic cells. The 220 kD CaBP has been found to be a component of a calcium-sensitive contractile fiber system which physically links the basal body complex to the nucleus in Chlamydomonas and related algae. In addition to its cytoskeletal localization in algae, evidence has been obtained that sequence-related and antigenically-related proteins to the 20 kD CaBP are associated with the spindle pole body in yeast cells and the centrosome of insight into the structure/function relationship of the 20 kD CaBP as a component of a contractile fiber system which appears to be distinct from actomyosin and microtubles based systems of motility and as a potentially conserved component of microtubule organizing centers of divergent structure in eukaryotic cells.